Deep Boundary Current Research Articles

Sampling-Dependent Transition Paths of Iceland–Scotland Overflow Water

Abstract In this note, we apply transition path theory (TPT) from Markov chains to shed light on the problem of Iceland–Scotland Overflow Water (ISOW) equatorward export. A recent analysis of observed trajectories of submerged floats demanded revision of the traditional abyssal circulation theory, which postulates that ISOW should steadily flow along a deep boundary current (DBC) around the subpolar North Atlantic prior to exiting it. The TPT analyses carried out here allow attention to be focused on the portions of flow from the origin of ISOW to the region where ISOW exits the subpolar North Atlantic and suggest that insufficient sampling may be biasing the aforementioned demand. The analyses, appropriately adapted to represent a continuous input of ISOW, are carried out on three time-homogeneous Markov chains modeling the ISOW flow. One is constructed using a high number of simulated trajectories homogeneously covering the flow domain. The other two use much fewer trajectories which heterogeneously cover the domain. The trajectories in the latter two chains are observed trajectories or simulated trajectories subsampled at the observed frequency. While the densely sampled chain supports a well-defined DBC, whether this is a peculiarity of the simulation considered or not, the more heterogeneously sampled chains do not, irrespective of the nature of the trajectories used, i.e., observed or simulated. Studying the sampling sensitivity of the Markov chains, we can give recommendations for enlarging the existing float dataset to improve the significance of conclusions about long-time-asymptotic aspects of the ISOW circulation.

The Role of Small‐Scale Topography in Modulating Eddy Scale in the Northern South China Sea

AbstractA high‐resolution nested model initialized with either smooth or synthetically generated rough topography is used to investigate the role of small‐scale topography in modulating eddy scales in the northern South China Sea. It is found that while adding small‐scale topography leads to negligible changes in the surface eddy scales and their seasonal cycle, it significantly reduces the bottom eddy scales by about 30%–40%. This reduction in bottom eddy scales is mainly contributed by three processes: wave generation due to flow interaction with rough topography and subsequent wave propagation into the ocean interior, forward energy cascade associated with processes such as nonpropagating form drag effect, and the influence of small‐scale topography on the deep boundary current. Our results highlight the importance of small‐scale topography in setting the eddy length scales particularly in the deep ocean.

Influence of terrigenous supply from the African continent on sedimentary CaCO3 distribution in the Southeast Atlantic Ocean

AbstractTerrigenous materials are transported to the ocean via various pathways, which can modify the weight contents of calcium carbonate (wt% CaCO3) in marine sediment and mask CaCO3 dissolution profiles, especially in ocean margins. Although the effect of terrigenous dilution on marine CaCO3 has long been established, there is still a lack of quantitative assessment on how the terrigenous input shapes sedimentary CaCO3 depth distribution features. In this study, we focus on the CaCO3 depth distribution patterns on the seafloor in the Southeast Atlantic Ocean. Based on an up‐to‐date compilation of surficial sedimentary CaCO3 content data and a simple conceptual model, the regional dilution effect of terrigenous supply from the African continent on the basinal sedimentary CaCO3 depth distribution is quantified. Our results show that the flux of noncarbonate materials needed to reproduce the carbonate distribution decreases with distance to the African continent. The noncarbonate flux to pelagic sediment in the Southeast Atlantic Ocean is on the same order of magnitude of the dust deposition rate, while the flux to marginal sediment is intensified as a consequence of the fluvial discharge. Moreover, enhanced dissolution of CaCO3 is found in marginal regions, which is due to faster rates of deep (boundary) currents than those in the pelagic ocean. These new insights help to distinguish the dilution impact from carbonate dissolution patterns and will advance research on land–ocean interaction and paleoclimate/environment reconstructions in ocean margins.

Overflow water pathways in the North Atlantic

As part of the international Overturning in the Subpolar North Atlantic Program (OSNAP), 135 acoustically-tracked deep floats were deployed to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) from 2014 to 2018. These water masses, which originate in the Nordic Seas, are transported by the deepest branch of the Atlantic Meridional Overturning Circulation (AMOC). The OSNAP floats provide the first directly-observed, comprehensive Lagrangian view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. The collection of OSNAP float trajectories, complemented by model simulations, reveals that their pathways are (a) not restricted to western boundary currents, and (b) remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower and along multiple export pathways. The characterization of these Overflow Water pathways has important implications for our understanding of the AMOC and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.

Transition Paths of North Atlantic Deep Water

Abstract Recently introduced in oceanography to interpret the near-surface circulation, transition path theory (TPT) is a methodology that rigorously characterizes ensembles of trajectory pieces flowing out from a source last and into a target next, i.e., those that most productively contribute to transport. Here we use TPT to frame, in a statistically more robust fashion than earlier analysis, equatorward routes of North Atlantic Deep Water (NADW) in the subpolar North Atlantic. TPT is applied on all available RAFOS and Argo floats in the area by means of a discretization of the Lagrangian dynamics described by their trajectories. By considering floats at different depths, we investigate transition paths of NADW in its upper (UNADW) and lower (LNADW) layers. We find that the majority of UNADW transition paths sourced in the Labrador and southwestern Irminger Seas reach the western side of a target arranged zonally along the southern edge of the subpolar North Atlantic domain visited by the floats. This is accomplished in the form of a well-organized deep boundary current (DBC). LNADW transition paths sourced west of the Reykjanes Ridge reveal a similar pattern, while those sourced east of the ridge are found to hit the western side of the target via a DBC and also several other places along it in a less organized fashion, indicating southward flow along the eastern and western flanks of the Mid-Atlantic Ridge. Naked-eye inspection of trajectories suggest generally more diffused equatorward NADW routes. A source-independent dynamical decomposition of the flow domain into analogous backward-time basins of attraction, beyond the reach of direct inspection of trajectories, reveals a much wider influence of the western side of the target for UNADW than for LNADW. For UNADW, the average expected duration of the pathways from the Labrador and Irminger Seas was found to be of 2–3 years. For LNADW, the duration was found to be influenced by the Reykjanes Ridge, being as long as 8 years from the western side of the ridge and of about 3 years on average from its eastern side.

Intruding gravity currents and their recirculation in a rotating frame: Numerical results

Using a set of direct numerical simulations (DNS) we investigate the circulation of a buoyancy driven experiment in a circular rotating tank. The initial density structure is a two-layer stable-stratification. The water is densified at 32 points at the upslope side of a circular incline and flows downward as a gravity current before it intrudes to the interior along the interface between the two layers. The water above recirculates to the densification points.When the buoyancy production is lower than the maximal Ekman transport, the gravity current proceeds in the laminar Ekman layer. In the opposite case, the water cascades down the incline in a no-stationary dynamics, turbulent transport becomes important and the boundary layer is thickened. The thermal wind relation applied to the tangential velocity averaged-along-the-slope predicts the density structure averaged-along-the-slope in the domain.When the densified water arrives at the interface it flows along the slope in the cyclonic direction, in a geostrophically adjusted deep boundary current. The distance of the boundary current from the slope is given by inertial overshoot of the Ekman current due to the self advection by the inertia. When this distance is small the boundary current is close to the slope and subject to boundary friction. In the opposite case the boundary current evolves detached from the boundary. The intrusion along the interface, in the (negative) radial direction, is slow, as the intruding distance is proportional to the square-root of time.The recirculation in the upper layer is close to a geostrophic equilibrium. Its quasi-geostrophic evolution is due to the compression of the water column, caused by the gravity and intruding currents, below. This leads to a large-scale anti-cyclonic circulation in the upper layer. When the associated geostrophic surface pressure gradient exceeds the pressured gradient due to the dense water on the slope, the Ekman transport is upslope.A special emphasis is put on the interactions of the three phases of the circulation: gravity-current, intrusion and recirculation. The circulation and the mixing of density and momentum in the gravity current determine the water-mass properties in the deep boundary current. Both determine the recirculation. The recirculation leads to an increasing anti-cyclonic circulation above the gravity current which, through the corresponding surface pressure gradient, reduces and finally blocs the downslope movement in the Ekman layer.

Bathymetric and regional benthic foraminiferal distribution on the Espírito Santo Basin slope, Brazil (SW Atlantic)

To understand the main environmental factors controlling the benthic foraminiferal distribution along continental slopes, and corroborate paleoenvironmental interpretations, we investigated benthic foraminiferal assemblages (total fauna >63 μm) and sedimentological data along six transects on the continental slope of the Espírito Santo Basin (ESB) between 18°20′ and 21°20′ S. Surface sediment samples (0–2 cm) were collected perpendicularly to the coast, from the upper (400 m) to the lower slope (3000 m). The density, taxonomic diversity and assemblage composition change with depth. The genus Globocassidulina dominates the upper and lower slope, whereas Bolivina is the most abundant taxon in the deepest lower slope. The differences in ecological preferences among the most abundant taxa allowed us to recognize five benthic foraminiferal groups, distributed in three main bathymetric sectors: upper, middle – lower, and lower slope. Group I (upper slope, 400 m) is characterized by the highest mean relative abundances of Globocassidulina rossensis, Trifarina spp. and Trifarina angulosa. Group II (middle – lower slope, 1000–1300 m) shows the highest mean relative abundances of Globocassidulina subglobosa, Bolivina albatrossi, Bulimina aculeata and Uvigerina peregrina. Group III (middle – lower slope, 1000–1300 m) is characterized by the highest mean relative abundances of Globocassidulina crassa, Gavelinopsis versiformis, Epistominella exigua and the unilocular group. Group IV (lower slope, 1900 m) shows the highest mean relative abundances of Alabaminella weddellensis, Bolivina inflata and E. exigua. Group V (lower slope, 2500–3000 m) is dominated by the genus Bolivina (B. lowmani, B. pseudoplicata and Bolivina spp). The most abundant taxa along the ESB slope are ecologically associated to the organic matter flux, bottom water oxygen concentration, and hydrodynamics conditions. The quantity and quality of food supply is the primary factor controlling the distribution of benthic foraminiferal assemblages along depth. Secondary factors include the properties of water masses, and intermediate and deep boundary currents. The increased fragmentation of benthic and planktic foraminifera tests (>125 μm) on the upper and middle – lower slope is related to abrasion during transport from shallower regions to the slope.

Deep Eastern Boundary Currents: Idealized Models and Dynamics

AbstractConcentrated poleward flows along eastern boundaries between 2- and 4-km depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these deep eastern boundary currents (DEBCs) using idealized GCM simulations, and we use a conceptual vorticity model for the DEBCs to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.

Deep Eastern Boundary Currents: Realistic Simulations and Vorticity Budgets

AbstractConcentrated poleward flows near the eastern boundaries between 2- and 4-km depth have been observed repeatedly, particularly in the Southern Hemisphere. These deep eastern boundary currents (DEBCs) play an important role in setting the large-scale tracer distribution and have nonnegligible contribution to global transports of mass, heat, and tracers, but their dynamics are not well understood. In this paper, we first demonstrate the significant role of DEBCs in the southeastern Atlantic, Indian, and Pacific Oceans, using the Southern Ocean State Estimate (SOSE) data assimilating product, and using high-resolution regional general circulation model configurations. The vorticity balances of these DEBCs reveal that, over most of the width of such currents, they are in an interior-like vorticity budget, with the meridional advection of planetary vorticity βυ and vortex stretching fwz being the largest two terms, and with contributions of nonlinearity and friction that are of smaller spatial scale. The stretching is shown, using a temperature budget, to be largely forced by resolved or parameterized eddy temperature transport. Strongly decaying signals from the eastern boundary in friction and stretching form the dominant balance in a sublayer close to the eastern boundary. The temporal variability of DEBCs is then examined, to help to interpret observations that tend to be sporadic in both time and space. The probability distribution functions of northward velocity in DEBC regions are broad, implying that flow reversals are common. Although the regions of the simulated DEBCs are generally local minima of eddy kinetic energy, they are still constantly releasing westward-propagating Rossby waves.

Dynamics of Deep Ocean Eastern Boundary Currents

AbstractThe long‐standing paradigm for the large‐scale time‐averaged ocean circulation in the world oceans includes intensified currents at the western boundary, and a much slower interior flow elsewhere. However, poleward deep boundary currents in the eastern limits of the deep South Pacific, Atlantic, and Indian Oceans have been observed repeatedly at 2‐ to 4‐km depth. They carry up to a third of the total deep ocean transports, implying a significant role in climate, yet their dynamics are still not well understood. Here we develop a theoretical understanding for these currents, using a hierarchy of realistic and idealized models, focusing on the South Pacific Ocean. The deep eastern boundary current there is shown to be driven by a traditional interior balance together with a narrow boundary layer scale near the eastern boundary, which exists only when stratification and topography are both included. A simplified semianalytical vorticity model is developed for the deep eastern boundary current.

A Deep Eastern Boundary Current Carrying Indian Deep Water South of Australia

AbstractIn the Southern Hemisphere, the ocean's deep waters are predominantly transported from low to high latitudes via boundary currents. In addition to the Deep Western Boundary Currents, pathways along the eastern boundaries of the southern Atlantic, Indian, and Pacific transport deep water poleward into the Southern Ocean where these waters upwell to the sea surface. These deep eastern boundary currents and their physical drivers are not well characterized, particularly those carrying carbon and nutrient‐rich deep waters from the Indian and Pacific basins. Here we describe the poleward deep eastern boundary current that carries Indian Deep Water along the southern boundary of Australia to the Southern Ocean using a combination of hydrographic observations and Lagrangian experiments in an eddy‐permitting ocean state estimate. We find strong evidence for a deep boundary current carrying the low‐oxygen, carbon‐rich signature of Indian Deep Water extending between 1,500 and 3,000 m along the Australian continental slope, from 30°S to the Antarctic Circumpolar Current southwest of Tasmania. From the Lagrangian particles it is estimated that this pathway transports approximately 5.8 ± 1.3 Sv southward from 30°S to the northern boundary of the Antarctic Circumpolar Current. The volume transport of this pathway is highly variable and is closely correlated with the overlying westward volume transport of the Flinders Current.

How does Subantarctic Mode Water ventilate the Southern Hemisphere subtropics?

AbstractIn several regions north of the Antarctic Circumpolar Current (ACC), deep wintertime convection refreshes pools of weakly stratified subsurface water collectively referred to as Subantarctic Mode Water (SAMW). SAMW ventilates the subtropical thermocline on decadal timescales, providing nutrients for low‐latitude productivity and potentially trapping anthropogenic carbon in the deep ocean interior for centuries. In this work, we investigate the spatial structure and timescales of mode water export and associated thermocline ventilation. We use passive tracers in an eddy‐permitting, observationally‐informed Southern Ocean model to identify the pathways followed by mode waters between their formation regions and the areas where they first enter the subtropics. We find that the pathways followed by the mode water tracers are largely set by the mean geostrophic circulation. Export from the Indian and Central Pacific mode water pools is primarily driven by large‐scale gyre circulation, whereas export from the Australian and Atlantic pools is heavily influenced by the ACC. Export from the Eastern Pacific mode water pool is driven by a combination of deep boundary currents and subtropical gyre circulation. More than 50% of each mode water tracer reaches the subtropical thermocline within 50 years, with significant variability between pools. The Eastern Pacific pathway is especially efficient, with roughly 80% entering the subtropical thermocline within 50 years. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 40 years for the Central Pacific mode water pool.

Baroclinic instability of axially symmetric flow over sloping bathymetry

Observations and models of deep ocean boundary currents show that they exhibit complex variability, instabilities and eddy shedding, particularly over continental slopes that curve horizontally, for example around coastal peninsulas. In this article the authors investigate the source of this variability by characterizing the properties of baroclinic instability in mean flows over horizontally curved bottom slopes. The classical two-layer quasi-geostrophic solution for linear baroclinic instability over sloping bottom topography is extended to the case of azimuthal mean flow in an annular channel. To facilitate comparison with the classical straight channel instability problem of uniform mean flow, the authors focus on comparatively simple flows in an annulus, namely uniform azimuthal velocity and solid-body rotation. Baroclinic instability in solid-body rotation flow is analytically analogous to the instability in uniform straight channel flow due to several identical properties of the mean flow, including vanishing strain rate and vorticity gradient. The instability of uniform azimuthal flow is numerically similar to straight channel flow instability as long as the mean barotropic azimuthal velocity is zero. Non-zero barotropic flow generally suppresses the instability via horizontal curvature-induced strain and Reynolds stress work. An exception occurs when the ratio of the bathymetric to isopycnal slopes is close to (positive) one, as is often observed in the ocean, in which case the instability is enhanced. A non-vanishing mean barotropic flow component also results in a larger number of growing eigenmodes and in increased non-normal growth. The implications of these findings for variability in deep western boundary currents are discussed.

Variability of neodymium isotopes associated with planktonic foraminifera in the Pacific Ocean during the Holocene and Last Glacial Maximum

The deep Pacific Ocean holds the largest oceanic reservoir of carbon which may interchange with the atmosphere on climatologically important timescales. The circulation of the deep Pacific during the Last Glacial Maximum (LGM), however, is not well understood. Neodymium (Nd) isotopes of ferromanganese oxide coatings precipitated on planktonic foraminifera are a valuable proxy for deep ocean water mass reconstruction in paleoceanography. In this study, we present Nd isotope compositions (εNd) of planktonic foraminifera for the Holocene and the LGM obtained from 55 new sites widely distributed in the Pacific Ocean. The Holocene planktonic foraminiferal εNd results agree with the proximal seawater data, indicating that they provide a reliable record of modern bottom water Nd isotopes in the deep Pacific. There is a good correlation between foraminiferal εNd and seawater phosphate concentrations (R2=0.80), but poorer correlation with silicate (R2=0.37). Our interpretation is that the radiogenic Nd isotope is added to the deep open Pacific through particle release from the upper ocean during deep water mass advection and aging. The data thus also imply the Nd isotopes in the Pacific are not likely to be controlled by silicate cycling. In the North Pacific, the glacial Nd isotopic compositions are similar to the Holocene values, indicating that the Nd isotope composition of North Pacific Deep Water (NPDW) remained constant (−3.5 to −4). During the LGM, the southwest Pacific cores throughout the water column show higher εNd corroborating previous studies which suggested a reduced inflow of North Atlantic Deep Water to the Pacific. However, the western equatorial Pacific deep water does not record a corresponding radiogenic excursion, implying reduced radiogenic boundary inputs during the LGM probably due to a shorter duration of seawater–particle interaction in a stronger glacial deep boundary current. A significant negative glacial εNd excursion is evident in mid-depth (1–2 km) cores of the eastern equatorial Pacific (EEP) which may suggest a stronger influence of NPDW return flow to the core sites and decreased local input in the EEP. Taken together, our Nd records do not support a dynamically slower glacial Pacific overturning circulation, and imply that the increased carbon inventory of Pacific deep water might be due to poor high latitude air–sea exchange and increased biological pump efficiency in glacial times.

Modification of turbulent dissipation rates by a deep Southern Ocean eddy

AbstractThe impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows.

On the deep water circulation of the eastern South Atlantic Ocean

The main flow patterns and transports of North Atlantic Deep Water (NADW) in the eastern South Atlantic and round southern Africa were inferred from two quasi-meridional WOCE hydrographic lines at 9°W and 5°E nominal longitudes, and from segments of other sections in the Atlantic-to-Indian Oceans transition region. An export of 11±4 Sv of NADW from the Atlantic was estimated near southern Africa inshore of the Subtropical Front. Most of this flow can be traced back across the Cape Basin to passages south of 28°S in Walvis Ridge, and farther upstream to an entry in the eastern basin at 20–25°S. Other transfers of NADW from the western boundary current to the eastern boundary were confirmed near the equator and at 22°S near the Namib Col of Walvis Ridge, yet a cyclonic recirculation of the former in the Angola Basin, and the weakness of the latter (4±2 Sv), make their direct contributions to the export of NADW minor ones. As another consequence, the estimated transport of the deep southward boundary current against the western continental slope of Africa shows important latitudinal variations (12±9 Sv at 7°30′S, 3±5 Sv at 22°30′S, 11±4 Sv at 35°S), with even a suggestion of its reversal in the northern Cape Basin. To the southwest of Cape Town, the deep boundary current is influenced by the temporal variability of the Agulhas retroflection region, and most of it separates from the slope to enter the Indian Ocean in the deep return flow of the Agulhas Current. Only a 2–3 Sv remnant against the continental slope was estimated at 30°E to the east of the Agulhas Plateau.

Chlorofluorocarbon‐derived formation rates of the deep and bottom waters of the Weddell Sea

The spreading and mixing of deep and bottom waters from the Weddell Sea (originating with potential temperature <0°C) are investigated through analysis of new and historical CFC measurements. We investigate the component that enters the southern Indian Ocean with a simple one‐dimensional advection‐diffusion model of the deep boundary current. This model does not require a condition of mixing solely with tracer‐free water to be imposed. As boundary conditions, we derive time histories for the CFC concentrations of newly formed Weddell Sea deep and bottom waters. A range of best fit velocities is obtained by determining which model velocities produce model tracer concentrations closest to measured concentrations. From this, we derive a formation rate of 3.2±1.5 Sv for the waters originating with temperatures lower than 0°C that subsequently enter the southern Indian Ocean. This compares to an upper limit for these waters of 5.7 Sv derived from the CFC‐11/CFC‐12 ratio. Whilst the former value is dependent on the highly idealized concepts of the model used, the latter is a more robust upper limit. Using the CFC‐11/CFC‐12 ratio measured at Vema Channel during the World Ocean Circulation Experiment cruise A23, an upper limit of 0.9 Sv is derived for the formation rate of waters colder than 0°C that enter the South Atlantic. Overall, an upper limit of 6.6 Sv is derived for the rate of formation of the deep and bottom waters of the Weddell Sea. A better representation is obtained by combining the upper limit to the South Atlantic component with the model‐derived southern Indian Ocean estimate; we thus estimate the formation rate of the deep and bottom waters in the Weddell Sea to be 3.7±1.6 Sv. Whilst the range obtained is large and the assumptions implicit in the calculations are potentially significant, we believe this more realistically represents the true formation rate than estimates derived using an imposed condition of tracer‐free mixing. The range can be narrowed in future as more data become available.

Sources and transport of the Deep Western Boundary Current East of the Kerguelen Plateau

East of the Kerguelen Plateau, a deep western boundary current in the Australian‐Antarctic Basin brings cold dense waters north from the margins of Antarctica. Geostrophic velocities referenced to acoustic Doppler current profiler data, both shipboard and lowered, suggest the flow is unidirectional throughout the water column with estimated northwestward transport below potential temperature 1°C of 28 ± 7 × 106 m³s−1 and 49 ± 9 × 106 m³s−1 respectively. Hydrographie and acoustic Doppler current profiler data show that the deep boundary current is supplied by northward flow emerging from the confluence of westward flow along the Antarctic continental slope and eastward flow of Weddell Basin waters through the Princess Elizabeth Trough to the south of the Kerguelen Plateau.

On deep western boundary currents

The Stommel and Arons [1960a, b] theory of the deep circulation of the world ocean does not directly address the physics of deep boundary currents. In the present paper a maximally simple steady state physics for deep western boundary currents is developed and integrated into the Stommel‐Arons model. This is accomplished by modifying the assumptions of the latter to include effective horizontal eddy viscous terms everywhere in the deep water, including in the interior. The resulting solution for the meridional velocity in a rectangular ocean on a beta plane is mathematically analogous to that of Munk [1950] for the wind‐driven circulation. In the interior the meridional velocity reduces identically to that of Stommel and Arons. Among other benefits, the solution eliminates the theoretical difficulties associated with the crossing of the equator by the deep western boundary current. As an initial test of the theory's consistency with observation, theoretical plots of pressure and meridional velocity as a function of longitude are qualitatively compared with hydrographic sections and inferred meridional velocity fields in the eastern and western basins of the abyssal North Atlantic. Also, the theoretical profile for the meridional velocity as a function of longitude is qualitatively compared with observations of longshore velocity as a function of offshore distance in the deep western boundary current of the North Atlantic Deep Water.

Hydrographic sections across the Atlantic at 7°30N and 4°30S

Transatlantic hydrographic sections along 7°30N and 4°30S, and shorter meridional ones along 35°W and 4°W in the intervening latitudinal range, provide a basin-wide description of the Atlantic water masses at their crossing of the equator. The water masses belonging to either the cold or warm segment of the global thermohaline cell enter the equatorial region mostly in the form of western boundary currents. The ways they leave it are more varied.The Ekman drift and a geostrophic western boundary current cause the export of near-surface water to the North Atlantic. A part of the southern Salinity Maximum Water, regarded as the shallowest warm water component, is thought to follow this route after experiencing strong property modification in the equatorial upwelling. The underlying South Atlantic Central Water divides into two northward paths, a direct one along the south American continental slope, hardly observed in the data because of an intense variability in the western half of the 7°30N line, and a longer one through the eastern basin, taken by water of the equatorial thermostad. There is no trace of such as eastern northward route for the Antarctic Intermediate Water, which is apparently forced northward from the equatorial region through the highly variable circulation of the western basin.The deep western boundary currents carrying southward the upper and middle components of the North Atlantic Deep Water experience a first partial shift to the eastern boundary on crossing the equator. At deeper levels, a part of the lower North Atlantic Deep Water also bifurcates eastward at the equator, but loses its identity through vertical mixing with the Antarctic Bottom Water in the equatorial fracture zones. The newly formed homogeneous bottom water proceeds eastward in the Guinea Basin, with further indication of an overflow into the Angola Basin.Beside the North Atlantic Deep Water, the deep layer of the equatorial region contains a lower-oxygen component, most clearly present between the middle and lower cores of the North Atlantic Deep Water. Previous results on this water are substantiated, namely, an arrival from the southeast, and northwestward crossing of the equator offshore from the deep western boundary current of northern water. A further northward progression of the southern water requires that the equatorial branching of the southward deep boundary current be only intermittent.A comparison of the temperatures along 7°30N in 1993 with those obtained at 8°N during the International Geophysical Year, 36 years before, reveals a net warming of the intermediate and upper deep waters, and cooling of the bottom water. This result is similar to that obtained at 24°N by other authors, yet there are signs of a southward propagation of a deep cold anomaly in the western basin, which had reached 24°N in 1992, but not yet 7°30N in 1993.